Add new make targets 'install' & 'uninstall'

This lets you install the compiled plugin files into
~/.local/share/gnome-shell/extensions/ and remove the files again
afterwards. This should be useful for people who download the source
and want to install it.
Add license file and boilerplate in code

According to the README, this project is licensed under GNU GPL v2, so I
added a license file for it and a description with copyright information
in the main extension file.